With the requirements for energy conservation becoming increasingly rigorous in various countries in the world, more rigorous requirements are put forward with respect to the efficiency and energy conservation of motors. In order to improve the efficiency of motors, their loss must be reduced. The loss of motors can be roughly divided into copper loss of stators and rotors, basic iron loss, mechanical loss and stray loss, among which copper loss and iron loss respectively account for about 40% and 20% of the total loss and are both related to the magnetic induction and magnetic permeability of electrical steel sheets, which are the materials used for manufacturing motors. Given that improving the magnetic induction and magnetic permeability of electrical steel sheets can help to reduce the copper loss and iron loss, the non-oriented electrical steel sheet featured by low iron loss and high magnetic permeability has become the preferred material for making high-efficiency motors.
Generally, Si, Al and other relevant elements are added to increase the electrical resistivity of materials and thereby reduce iron loss. For example, the Japanese patent JP-A-55-73819 discloses that, by adding an appropriate amount of Al and adjusting the annealing atmosphere, the internal oxide layer on steel sheet surface can be reduced, thereby achieve excellent magnetic performance. Similarly, Japanese patents JP-A-54-68716 and JP-A-61-87823 disclose that, adding Al or REM or optimizing the cooling rate of annealing can also improve magnetic performance.
However, adding only Si, Al and other relevant elements, or simultaneously employing corresponding process optimization to improve magnetic performance can achieve a very limited effect, because, as is well known, adding Si and Al would lower the magnetic induction and magnetic permeability of electrical steel sheets and thus reduce the efficiency of motors.
The U.S. Pat. No. 4,545,827 discloses a method for manufacturing a non-oriented electrical steel sheet featured by low iron loss and high magnetic permeability, wherein C content (wt %) is adjusted to control the carbide precipitation of products and the temper rolling technique is adopted to obtain 3.5-5.0 ASTM ferrite grain and easily magnetizable texture ingredients. However, the ingredient system of the patent is characterized by low Si and high C, and high C content may easily lead to magnetic aging and increased iron loss.
The U.S. Pat. No. 6,428,632 discloses a non-oriented electrical steel with low anisotropy and excellent processing property and applicable in high-frequency areas. The patent requires that the properties of steel sheets to satisfy the conditions of formulas B50 (L+C)≧0.03W15/50(L+C)+1.63 and W10/400(D)W10/400(L±C)≦1.2, so as to manufacture motors with high efficiency (above 92%). However, the non-oriented electrical steel manufactured with the patent technology is mainly used for high-frequency rotary motors, which require high production cost and thus not applicable for ordinary industrial motors.
Therefore, developing non-oriented electrical steel sheets with low production cost, low iron loss and high magnetic permeability and applicable for industrial motors has presented a broad market prospect. For this purpose, the present inventors have designed the research protocol on the basis of the following idea: By controlling the air cooling time and final rolling temperature of the hot rolling process and coarsening the inclusions in the steel, both the recrystallization percentage and grain size of the hot-rolled sheet are increased, so as to obtain non-oriented electrical sheets with low iron loss and high magnetic permeability and thereby produce non-oriented electrical steel sheets which can be used to improve the efficiency of ordinary industrial motors as well as high-efficiency and super high-efficiency industrial motors. Particularly, the present invention relates to a non-oriented electrical steel sheet which is applicable for manufacturing industrial motors with a working magnetic flux density of 1.0˜1.6 T and can improve the efficiency of the motors by 1%.